tests/vm: do not specify -bios option

tests/vm: do not specify -bios option

When running from the build tree, the executable is able to find
the BIOS on its own; when running from the source tree, a firmware
blob should already be installed and there is no guarantee that
the one in the source tree works with the QEMU that is being used for
the installation.

Just remove the -bios option, since it is unnecessary and in fact
there are other x86 VM tests that do not bother specifying it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>